MQE: initial query planner implementation

[charleskorn]

What this PR does

This PR introduces a query planner for MQE. At present, it takes the original PromQL expression, translates it to a query plan, then converts that to operators for evaluation. No significant optimisations (eg. common subexpression elimination) are applied to the query plan as part of this PR.

The main parts of this PR are:

• the QueryPlanner type in pkg/streamingpromql/planning.go: the main entrypoint to the query planning functionality, responsible for creating query plans from PromQL expressions
• the QueryPlan type in pkg/streamingpromql/planning/plan.go: represents a query plan
• the Node type in pkg/streamingpromql/planning/plan.go: represents a node in a query plan, and provides methods to examine, manipulate and convert the node to an operator
• the EncodedQueryPlan and EncodedNode types in pkg/streamingpromql/planning/plan.proto: represent a query plan transformed into a form suitable for serialization / deserialization
• two simple AST optimisation passes in pkg/streamingpromql/optimize/ast

I've run our existing benchmarks with query planning enabled and disabled. For non-trivial cases, the cost of running planning is noise compared to the cost of evaluating the query itself.

There are a number of improvements I'd like to introduce as follow-up PRs:

• metrics for planning latency
• investigate using code generation for methods like EquivalentTo(), Details(), Children(), SetChildren() and ChildrenLabels()
• moving the responsibility for converting a node to an operator elsewhere: the current design will make passing configuration (eg. cache connection parameters) difficult

I'll also introduce the first optimisation that builds on this as a follow-up PR.

I'll add a changelog entry when this PR is nearly ready to merge.

Which issue(s) this PR fixes or relates to

(none)

Checklist

• Tests updated.
• [n/a] Documentation added.
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX].
• [n/a] about-versioning.md updated with experimental features.

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